1994 Volume 34 Issue 1 Pages 17-23
Bake hardening properties of Ti-bearing ultra low carbon steels are affected by the precipitation behavior of TiN, MnS, TiS and Ti4C2S2. Thus the methods of isolation and determination of these precipitates have been developed and applied to the study on the precipitate change with reheating of the steels.
All precipitates, TiN, MnS, TiS and Ti4C2S2 were extracted by MS electrolysis. TiN was extracted by 10% (v/v) bromine-methanol. MnS was isolated from the residues (extracted by MS electrolysis) by 50°C HCl treatment. TiS in the 50°C HCl treated residues was decomposed by 10% (m/v) iodine-methylacetate and Ti4C2S2 remained. Mn, Ti and S in the residues were determined directly by X-ray fluorescence method. N was determined by neutral titration method. The determined compositions of precipitates except TiS almost corresponded with their stoichiometries. TiS was determined to TiS1.3.
Whole nitrogen precipitated as TiN and the amount did not change with reheating temperatures between 950 and 1250°C. Total amount of the sulfides such as MnS, TiS and Ti4C2S2 changed scarcely with reheating temperatures, but the amount of each sulfide changed largely. Precipitates in the steels reheated below 1050°C were mainly Ti4C2S2. It decomposed and transfomed into TiS and MnS with raising the reheating temperature, and decreased drastically at 1150°C. The precipitates above 1150°C were mainly MnS and TiS, and the fraction depended on the ratio of Ti to Mn concentration in the steels.